1. Field of the Invention
The present invention relates to a remote wireless communication system such as a Bluetooth system and a communication method thereof, and more particularly, to a wireless communication system capable of guaranteeing both high throughput and fairness of wireless communication devices during data transmission and reception between one wireless communication device and plural wireless communication devices, and a communication method thereof.
2. Description of the Prior Art
Bluetooth is a code name of a wireless data communication technology that is applied in the fields of electronics and telecommunications, networking, computing and consumables. The Bluetooth technology can replace various cables that otherwise would be required to be connected between the communication devices for each wireless communication connection, and enables wireless communication within short distances. For example, the Bluetooth technology applied in a mobile telephone and a laptop computer enables connection of the mobile phone and the laptop computer without requiring cables. Almost all digital devices, including a printer, a personal digital assistant, a desktop PC, a facsimile machine, a keyboard and a joystick, can be a part of the Bluetooth system.
Generally, Bluetooth operates with a maximum data transmission speed of 1 Mbps and at a maximum transmission distance of 10 m. As ‘1 Mbps’ is a data transmission speed within the industrial scientific medical (ISM) frequency band of 2.4 GHz which can be used by a user without a license, the transmission speed can be easily achievable at low cost. The maximum transmission distance is also set as 10 m in consideration of the fact that 10 m is sufficient a distance in the office for a mobile device carried by a user and a PC on the desk to communicate.
Having been designed to operate in a radio frequency environment that is laden with noises, Bluetooth enables stable data transmission and reception even at a noisy wireless frequency by using frequency hopping with the hopping rate of 1600 hops per second. The frequency hopping is often called as a frequency hopping spread spectrum (FHSS) scheme. In the FHSS scheme, a given frequency band is segmented into lots of hopping channels, so that when a firstly modulated signal (intermediate frequency) from a transmitter is converted into the radio frequency band of 2.4 GHz, the signals are appointed to different hopping channels in a predetermined order. Since signal appointing channels change at rapid speed, influences by multi-channel interference and narrow bandwidth impulse noise can be lessened. At a receiver's end, the original signals are recovered as the signals distributed to the hopping channels are connected with each other in the same order as at the transmitter's end. IEEE 802.11 uses 79 hopping channels, and each of the hopping channels is arranged at a 1 MHz interval. At least a 6 MHz interval is set between two temporally neighboring hopping channels so that inter-channel interference can be avoided when the signals are allotted with hopping over many channels. Speed of changing hopping channels (i.e., hopping rate) is also set to be more than 2.5 times per second.
In addition to one-to-one connection, the Bluetooth system also supports one-to-multi connection. As shown in the Bluetooth system of FIG. 1, there may be several piconets constructed and connected, while each piconet is characterized by the respective frequency hopping priorities. The piconet is one unit of Bluetooth system, in which more than one slave is connected to a single master. One piconet has one master, and can have up to 7 slaves. For example, FIG. 1 shows piconets A and B, piconet A having a master 10 and three slaves 13, and piconet B having a master 10 and one slave 13. The master device decides overall properties of the channels within the piconet. The Bluetooth device address (BD_ADDR) of the master determines frequency hop sequence and channel access code. In other words, the clock of the master determines a phase of hop sequence and sets timing. Also, the master controls traffic in the channels. Any digital device can be a master, and the role of a master and a slave can change after a piconet is established.
Basically, a master device and a slave device perform bi-directional communication by time division duplex (TDD) in the unit of 1 hopping slot (625 μs= 1/600 sec). A plurality of piconets connected in a certain structure is called a ‘scatternet’.
FIG. 2 is a view showing the communications between the master and the slave by TDD. Referring to FIG. 2, the length of each channel allotted to a time slot is 625 μs. The number of time slots is decided according to the Bluetooth clock of the piconet master. The master and slave may selectively transmit packets by the time slots. That is, while the master transmits packets only in even-numbered time slots, the slave transmits packets only in odd-numbered time slots. Packets that are transmitted either by master or slave must be realized within the 5 time slots. A ‘packet’ refers to the unit of data being transmitted in the piconet channel.
When more than 2 slaves access 1 master in the piconet, the master assigns temporary 3-bit addresses to the slaves for later use to distinguish the slaves when they are activated. In other words, all the packets being exchanged between the master and the slaves carry AM_ADDR. The AM_ADDR is represented as the member address, which identifies active members participated in the piconet. Not only in the packet transmitted from the master to the slave, the AM_ADDR is also used in the packet transmitted from the slave to the master. The AM_ADDR is given up if it is assigned when the slave is not connected to the master, or when the slave is in a park mode. Then a new AM_ADDR is required to be assigned when the slave is re-connected to the master. One piconet has no more than one master and seven slaves because the AM_ADDR that the master assigns to the active slaves is set with 3-bits in length. In other words, since the address “000” among maximum 8 addresses is used for broadcasting from the master to the slave, the rest of the addresses, i.e., the 7 addresses from “001” to “111” can be used.
When one master transmits and receives data with more than two slaves in the piconet, the master segments time slot at uniform intervals and allocates the respective time slots to the slaves, and transmits and receives data through the allocated time slots. Accordingly, data collision is avoided.
Usually, the master transmits and receives data with the respective slaves wirelessly using the round-robin polling.
FIG. 3 is a view showing data transmission/reception by the round-robin polling in the case where one master communicates with 3 slaves. Referring to FIG. 3, the round-robin polling only allows the slave polled by the master to send the data. In other words, the master can transmit data in the even-numbered slots, while the slaves, which are polled by the master, can transmit data in the adjacent odd-numbered slots. The rest slaves, i.e., unpolled slaves, may not transmit the data in the corresponding slots. In this case, the master sequentially polls slave 1, slave 2 and slave 3, by which the respective master-slave pairs transmit and receive data in the transmission rate that is one third of the overall transmission rate.
According to the round-robin polling, there is no problem when the master-slave pairs respectively have the same transmission rate. However, if the transmission rate of each master-slave pair is not identical, system efficiency degrades. In other words, since each allotted slot is used for the exchange of POLL-NULL packets regardless of whether one master-slave pair has no, or less data transmission than the other master-slave pairs, slot wastage occurs, and as a result, overall performance of the system degrades.